3D Ultrasonic imaging method

ABSTRACT

A probe for electronic volume data acquisition using ultrasound incorporates a plurality of transducer elements arranged in a two dimensional array having an azimuth direction and an elevation direction. The transducer elements have a first element size in the azimuth dimension and a second element size in the elevation dimension. At least one of the first and second element sizes is at least twice a characteristic wavelength of a waveform used to drive the array of transducer elements, where the characteristic wavelength is defined as the wavelength corresponding to a center frequency of the waveform. Image data is generated in a scanning process using a CAC-BF technique in an azimuth dimension and/or an elevation dimension, to form an ultrasound image line, image plane, or image data cube.

FIELD OF INVENTION

This invention relates to ultrasound imaging systems. More particularly,this invention relates to methods and devices for three-dimensionalimage acquisition. The devices and methods are also suitable for 2D and4D ultrasound systems. The invention is particularly, but notexclusively, useful for medical diagnoses and treatment. The devices andmethods of the present invention are useful components of practicalhigh-quality real-time 3D ultrasound systems with fully electronicvolume data acquisition.

BACKGROUND OF THE INVENTION

Two-dimensional (2D) ultrasonic probes are necessary to supportthree-dimensional (3D) electronic, volume data acquisition for manyclinical applications. State-of-the-art one-dimensional (1D and 1.5D)probes which electronically scan only in azimuth provide the 2Dultrasound images (azimuth and range) which are commonly used today. 2Dprobes scan electronically in elevation as well as azimuth, to provide athree dimensional data cube (azimuth, elevation and range) which can beprocessed using image processing software to produce a variety of imageformats. These formats include conventional planar images, planar imagesat arbitrary scan planes, as well as representations such as surfacerendering and orthographic presentations. Four-dimensional (4D)representations include 3D animations where the 3D rendering is updatedin time.

Two-dimensional sensors are employed in other imaging modalities such asCT-scanners, and in other fields such as radar; and hence are wellunderstood conceptually. Practical difficulties arise with theultrasound modality due to the small, elemental feature size (fractionsof a mm) and the large number of channels typically needed. Thesedifficulties have stalled the introduction of fully electronic, 2D,ultrasonic probes.

Ultrasound systems today make use of a variety of 1D and 1.5D ultrasonicarrays. A 1D array has a fixed elevation aperture which is focussed at afixed range, and is usually realized with a mechanical lens of sorts. A1.5D array, on the other hand, has a variable elevation aperture,shading and focussing, but they are symmetric about the centerline ofthe array.

1D array transducers contain several tens or even hundreds of elementstypically arranged linearly. The transducer elements 10, 12 may bearranged on a straight line (linear array) or a curved line (curvedlinear array or simply curved array) as shown in FIGS. 1A and 1B,respectively. The operation of a linear array or curved array aresimilar, the main difference being that the image expands with range(depth) for the curved array. A typical linear or curved array couldhave anywhere from 64 to 512 (or more) elements, depending on the costand the application. The azimuthal spacing of elements is typicallybetween half a wavelength and one wavelength. The elemental size in theelevation dimension is much larger, typically tens of wavelengths. Theoperating frequency is typically somewhere between 2 MHz to 20 MHz,depending on the clinical application.

Let's consider an example, where a 7.5 MHz curved array of the typeshown in FIG. 1B has 256 transducer elements 12 in azimuth spaced by onewavelength; and the dimension of an element in elevation is, say, 40wavelengths. At 7.5 MHz, the wavelength, λ, in tissue is about 0.2 mm.Therefore, the array spans about 51 mm in azimuth and 8 mm in elevation.

A narrow beam is created in the azimuth dimension by focussing thetransmitted and receive energy along a particular beam or scan line 14,16, as illustrated in FIG. 2A and FIG. 2B. Scanning is performed inazimuth (i.e. in a single elevation plane) using one of two schemes,sequential scaring or phased-array scanning. With sequential scanning,any given beam line is offset from all of the other beam lines in theazimuth direction. If the array is linear (rather than curved), the beamlines 16, 18 are parallel to one another (FIG. 3A). With reference toFIG. 2B, the central beam line 16 that is illustrated is shifted(offset) to the left or right with different offsets 20 to create a setof beam lines 22 that spans the region or volume to be imaged, asillustrated in FIG. 3A. Phased-array scanning, on the other hand, isachieved by rotating the central beam line 24 illustrated in FIG. 3B inazimuth, to the left and to the right, by a set of angular offsets 26.The beam lines 28 of the resulting set 30 of beam lines intersect at acommon apex 32 (which may actually occur behind the array), and separatefrom each other as a function of range, as illustrated in FIG. 3B.

Premium probes generally employ wideband waveforms to achieve the fineresolutions needed in range. As a result, beamforming is done byadjusting time delays (in the narrowband waveform case, phases areadjusted rather than time delays) at each element used on transmit andreceive. For a given pulse, a focal point is set along the rangedimension. Appropriate time delays are used on the elements involved intransmission, so that their respective acoustic energy arrives at thespecified focal range, along the specified beam line, at the same time.As a result, the waveform is said to be focussed at this point. Onreceive, time delays are dynamically applied to the elements involved inreception, to focus the received energy at each range.

Generally speaking, focussing is needed only in the near field of thearray, where the ultrasound wave cannot be assumed to be planar, as itis in the far field. If one looks closely at the effect of thisfocussing operation in the azimuth and elevation spatial dimensions, onenotices a difference. In azimuth, numerous transducer elements areavailable, each with a respective time delay to adjust dynamically withrange on receive. The result is the azimuth resolution of the beam canbe generally maintained uniformly with range as illustrated in FIG. 2Afor a 1D linear array. There are no delays to adjust in elevation,however. As a result, a typical, fixed, lens-like beam pattern resultsin the elevation dimension, with the best elevation resolution occurringat the transmit focal point (in range), and with a degradation of theelevation resolution as one moves away from this focal point in range.This effect is also illustrated in FIG. 2A. The image plane thickness(i.e. in the elevation dimension) in effect varies with range for a 1Dlinear array.

The 1.5D array provides a solution to the image thickness problem, andtherefore produces higher-quality, planar images than the 1D array(Wildes, D. G., et al., “Elevation Performance of 1.25D and 1.5DTransducer Arrays”, IEEE Transactions on Ultrasound, Ferroelectronicsand Frequency Control. Vol. 44, No. 5, September 1997, pp. 1027 to1036). By using multiple rows of elements in the elevation dimension, asillustrated in FIG. 2B, multiple elevation lenses can be effected, eachfocussed at a different focal range. This is achieved by varying thetime delays (through switching or otherwise) applied to the elevationelements while the acoustic signals are being received. In addition, alens is typically used in the elevation dimension to help control theelevation focus. The net effect is that the elevation thickness(resolution) is maintained with range, thereby improving image quality.This is illustrated in FIG. 2B.

In a typical 1.5D array, each element might be λ×4λ (i.e. azimuth byelevation) in dimension. For an array with 128 elements per row and 8rows of elements, the elevation dimension is 32λ or 6.4 mm and theazimuth dimension is 128λ or 25.6 mm at 7.5 MHz.

Consider the linear 1.5D array shown in FIG. 3A, containing 256 elements34 in azimuth. Now 128 sequential beams 18 are typically used to form arectangular, azimuthal image plane by scanning in azimuth as illustratedin the figure. Typical transducer dimensions for this state-of-the-artarray are also indicated. (Note: only 16 columns of elements 34 areshown in FIG. 3A, for simplicity, where in fact, 256 elements arerepresented in the azimuth dimension).

A state-of-the-art array with λ/2 spacing in azimuth to supportphased-array scanning is illustrated in FIG. 3B . This type of arrayproduces pie-shaped images in contrast to the rectangular imagesproduced using sequential arrays.

Unlike 1.5D arrays which are commonly found in premium ultrasoundsystems, 1.75D arrays are not yet in use in commercial systems (PuyunGuo, Shikui Yan and Quing Zhu, “Elevation Beamforming Performance of a1.75D array”, IEEE 2001 Ultrasound, Ferroelectronics and FrequencyControl Conference). 1.75D arrays are like 1.5D arrays, except there isno symmetry constraint. As a result, it is possible to provide a littlebit of elevation steering. However, due to the large element size inelevation (several wavelengths), grating lobes become serious if theelectronic scanning is significant (Puyun Guo, Shikui Yan and Quing Zhu,“Elevation Beamforming Performance of a 1.75D array”, IEEE 2001Ultrasound, Ferroelectronics and Frequency Control Conference).

Interest in 3D Ultrasound is growing and all major ultrasound companiesare paying attention. There are two ways that scanning is currentlyperformed: sequential scanning and phased array scanning. It is commonknowledge to those skilled in the art that if one conventionally-extendsa 1D phased array (typically with λ/2 element spacing) to two dimensions(of equal size), or a 1D sequential array (typically with λ elementspacing) to two dimensions, then data cubes could be acquired by 2Dscanning, and the fine (e.g. an F number of 2, denoted herein as F/2)azimuth resolution currently available extends to elevation as well. Twofundamental difficulties, however, arise:

1. the cost is prohibitive;

2. the frame-time to acquire a 3D volume is far greater than the time ittakes to acquire a 2D image.

Consider extending a linear array with 256 elements (maximum of 128 usedon receive) to two dimensions. The number of elements increases to256×256=65,536. Transducer design/fabrication is very difficult, if notimpossible, today. The number of receiver channels would also increaseby a factor of 128 in order to provide the same resolution in bothdimensions, all else being equal, while not increasing the number ofshots (and hence acquisition time) needed per vector. Since system costis proportional to the number of channels, the resulting cost isunaffordable.

Finally, it takes longer to acquire the data cube (as compared to thetens of milliseconds needed to acquire a conventional 2D image plane)since there are many more beams needed to interrogate the volume. Atleast 128×128=16,384 beams are needed, for each transmit focal range,with about 100 μs two-way time needed for each shot (this assumes a 10kHz firing rate and a 7 cm depth needed). For two focal ranges, thisimplies an acquisition time of 3.2 seconds, assuming the number ofchannels available equals the number of elements used in the beamformer.

The aforementioned difficulties require practical trade-offs and novelsolutions if 2D arrays supporting 3D electronic, volume data acquisitionare to be an affordable reality.

Additional Prior Art

Many engineers have attempted to solve the aforementioned difficultiesin order to help make 3D ultrasound imaging an affordable reality. Someof the more relevant approaches with respect to the current inventionsare discussed below.

As a result of the complexities associated with 2D electronic scanning,some engineers have proposed the use of mechanical scanning in theelevation dimension. That is, a conventional, 1D linear array is used toprovide the conventional, B- or C-mode, range-azimuth, planar image; butit is moved up and down quickly using mechanical means such as a motor,to acquire successive planar images at a set of elevation positions. InU.S. Pat. No. 6,106,471 “Procedure for an Examination of Objects by theMeans of Ultrasound Waves”, Wiesauer, Fosodeder and Gritzky describesuch an approach. The mechanical movement in the elevation dimension isdone automatically and continuously using a motor in the 2D probehousing. As focussing scan lines requires precise, a priori knowledge ofthe location of the 1D array elements with time, the quality of 2D and3D imagery produced using this approach is limited by the accuracy ofthe mechanical movements in the elevation dimension.

In Ultrasonic Blanket with CAC and SCA patent application, U.S. Ser. No.09/514,928, filed 28 Feb. 2000, 3D volume data aqcuisition and focussingof beams using active transducers is described. A singular, rigidcarrier structure constructed using scalar transducer elements arrangedin the likeness of an array is disclosed. Signal transmission aperturesand data gathering apertures are formed and used to electronically scandesired regions and electronically acquire 3D volumetric data; wherecoherent aperture combining (CAC) is used to combine the structural datafrom multiple data gathering apertures, thereby increasing the size ofthe effective data gathering apertures employed, and thereby increasingimage resolution. Both monostatic (on pulse one, transmit and receiveout of aperture one, on pulse two, transmit and receive out of aperturetwo) and bistatic (transmit from one aperture and receive simultaneouslyon two or more apertures) operations are disclosed. Also disclosed isthe use of 1.5D and 1.75D array technology to form a 2D array andeffectuate volume data acquisition by scanning in azimuth and elevation.

In U.S. Pat. No. 6,482,160 “High Resolution 3D Ultrasound Imaging SystemDeploying a Multidimensional Array of Sensors and Method forMultidimensional Beamforming Sensor Signals”, Stergiopoulos andDhanantwari describe an adaptive beamforming method which can be used toprocess sensor signals received on a 2D array of sensors in order togenerate a high resolution, 2D beam response for each of a set of beamdirections defined by azimuth-elevation angle pairs. The methoddescribed in the specification is applicable to the case where theimaged object is in the far-field of the 2D array of sensors, andrelates only to processing techniques to be used on receive. It isassumed that a single, low-gain, transmit sensor (e.g. anomni-directional transducer element) is located away from the 2D arrayof sensors and illuminates the entire region being imaged, causing theultrasound energy to reflect from the object towards the receivingarray. The inventors exploit adaptive beamforming algorithms to increasethe spatial resolution otherwise unobtainable from the receive array,had conventional, linear beamforming techniques such as the discreteFourier transform been used. In theory, adaptive algorithms can estimatethe noise process competing with the desired signal associated with eachbeam, and use this information to adapt the receive beam so as to bettersuppress the noise. Adaptive and linear beamforming techniques are wellknown to those skilled in the art. The inventors acknowledge that if theassumed noise characteristics are inaccurate, performance of theadaptive beamformer will degrade significantly and may even result incancellation of the desired signal. Furthermore, implementing adaptivealgorithms directly on the full array of sensor data-requires verysignificant computational resources; and convergence of the adaptivesolution requires significant training data. To mitigate these practicaldifficulties, the inventors propose a partially adaptive beamformerwhich reduces the number of adaptive degrees of freedom (DOF) bypreprocessing the array sensor data using conventional Fourierbeamforming. The partially adaptive beamformer, for the case of a 2Darray, begins by dividing the array into smaller subapertures, each ofwhich is a 2D array. For each subaperture, a 2D conventional beamformeris implemented, which, for computational efficiency, is organized as acascade of two 1D beamformers. For example, each row of thesubaperture's sensor data would be processed using a 1D azimuth Fourierbeamformer; and then the resulting column of azimuth-processed datawould be operated on by a 1D elevation Fourier beamformer. This cascadedapproach, known to those skilled in the art, only applies for the caseof far-field imaging. Finally,, adaptive beamforming is performed on theresulting, conventionally-beamformed subaperture signals, by adaptivelyprocessing those respective subaperture signals which wereconventionally-beamformed to the same azimuth-elevation angle direction.

In U.S. Pat. No. 6,419,633 “2D Ultrasonic Transducer Array for TwoDimensional and Three Dimensional. Imaging”, A. Robinson, B. Robinsonand Detmer describe a particular implementation of an electronic 2Darray. The inventors disclose an electronic 2D transducer array that canbe configured or switched to provide both 2D arrays and 1D arrays for 3Dand 2D imaging, respectively. A variety of particular element switchingand summing circuits are disclosed to combine rows and columns ofelements as needed for each supported array configuration. By thesedesigns, the inventors intend to limit the total number of signal leadscoming out of their transducer (and by implication, the number ofdigital receiver channels in the ultrasound system). This objective isachieved primarily by using a sparse 2D array on receive, with admittednegative implications on array sensitivity and grating lobes. In theexample, 19-by-19 2D transducer array used for illustration, only 100signal leads are needed to support the sparse 2D array configuration,rather than 361 leads if all elements were used. They also intend that2D image quality is not degraded, as compared to conventional 1D arrayswhich are optimized for this purpose. This objective is met byconfiguring the 2D transducer array as a fully populated, 1D array.

In U.S. Pat. No. 6,238,346 “System and Method Employing Two-dimensionalUltrasound Array for Wide Field of View Imaging”, inventor Masondiscloses a 2D rectangular transducer array which scans an elongatedsector volume using fewer transducer elements than in prior art systems,while avoiding sidelobe anomalies. The switching circuitry forms anumber of small subarrays, where each subarray spans the entireelevation dimension and includes a contiguous subset of azimuthelements. Each subarray is shifted from the next in azimuth by oneelement. The 2D array provides phased-array scanning in the shorterelevation dimension using all of the elements in each subarray.Conventional arrays, on the other hand, perform phased-array scanning onthe longer azimuth dimension. The disclosed 2D array scans in azimuth bystepping through subarrays, one at a time, and beamforming so as toproduce scanlines normal to the face of the subarray. With this design,fewer elements are required along the elevation dimension because it iskept deliberately small, to produce an elongated sector volume. Inazimuth, fewer elements are needed because the elements are spacedfurther apart (as much as 2λ) since phased-array scanning is not used inthis dimension. In other words, the 2D array is sparse in the azimuthdimension. The transducer elements used are square in shape and lessthan or equal to 0.75λ in size.

OBJECTS OF THE INVENTION

An object of the present invention is to provide ultrasound imagingtechnology which may be incorporated in practical, affordable,high-quality, 3D ultrasound imaging systems which are clinically useful,and which exploit 3D, electronic, volume data acquisition.

Another object of the present invention is to provide 3D ultrasoundsystems using state-of-the-art elemental transducer technology (e.g.using 1.75D transducer technology), switching, multiplexer and cabletechnology, and real-time signal processing technology such as ASICbeamforming and filtering hardware.

The most significant cost and complexity in premium 2D ultrasoundsystems relates to the receive electronics, the cost of which isproportional to the number of digital receive channels. Foraffordability therefore, another object of the present invention is tokeep the number of digital receive channels in the contemplated 3Dultrasound systems similar to that provided in current premium 2Dultrasounds.

A further object of the present invention is that the 2D probe apparatusand beamforming methods disclosed provide high-quality volume data whichcan be used to generate high-quality 3D imagery (e.g. 3D surfacerendering or orthographic presentations) as well as 2D imagery ofsimilar or better quality than premium, state-of-the-art, 2D ultrasoundsystems currently produce. High-quality imagery is characterized byazimuth, elevation and range resolutions equal to or better than thatprovided by premium 2D ultrasound systems, as well as grating lobe beamresponses (or sidelobe responses for narrowband systems) similar to thatprovided by premium 2D systems.

It is another object of the present invention to provide azimuthresolution that is significantly better than that of a premium 2Dultrasound system using the same azimuth aperture.

It is another object of the present invention to provide means toproduce elevation resolution significantly better than that provided bypremium 1.5D arrays in use today.

It is another object of the present invention to providefully-electronic, 3D volume data acquisition to support rapid andaccurate interrogation of volumes combined with highest-quality, 3Dimage formation.

In order to maximize the clinical utility of the 3D ultrasound systemcontemplated with the present invention, it is an object of theinvention to minimize the 3D volume acquisition time to sustain thehighest 3D frame rates without significantly sacrificing affordabilityor image quality. For example, it is an object of the present inventionis that meaningful, high-quality 3D data cubes can be electronicallyacquired in a fraction of one second.

Another object of the present invention is to provide 2D probe andbeamforming technology that can be manufactured in a conformal formfactor, and be used as a building block (i.e. providing transmissionapertures and data gathering apertures) in ultrasound medical imagingsystems, exemplarily as described in U.S. Pat. No. 5,666,953, U.S. Pat.No. 5,871,446, U.S. Pat. No. 6,023,632, U.S. Pat. No. 6,319,201, andU.S. Pat. No. 6,106,463.

Another object of the present invention is to provide a compact anddeployable 3D ultrasound system of size, weight, power, and form-factorsimilar to conventional 2D ultrasound systems.

Yet another object of the present invention is to increase the imagequality of light-weight, portable, 2D ultrasound imaging systemsemploying fewer receive channels than premium 2D ultrasounds, withoutappreciably increasing the size or cost of such improved systems.

Another object of the present invention is to provide an ultrasoundsystem for 3D imaging of the carotid artery, providing improvements insafety and accuracy over current diagnostic methods.

A further object of the present invention is to provide ultrasoundtechnology permitting a standard 2D ultrasound medical procedure to becarried out more quickly and hence more safely.

An additional object of the present invention is to provide ultrasoundtechnology enabling a relatively unskilled medical practitioner theability to perform an ultrasound medical procedure.

Further objects of the invention will be apparent from the drawings anddescriptions herein. It is to be noted that each object is achieved byat least one embodiment of the present invention. However, it is notnecessary that any given embodiment achieve all of the objects of theinvention.

SUMMARY OF THE INVENTION

The present invention addresses the above-mentioned difficulties byemploying innovative approaches which together provide a practicalsolution to ultrasound imaging systems employing 2D ultrasonic arrayswhich support electronic, 3D volume data acquisition and beamforming.

The present invention is directed in part to a probe for electronic, 3Dvolume data acquisition using ultrasound, comprising a plurality oftransducer elements arranged in a two dimensional array having anazimuth dimension and an elevation dimension. The transducer elementshave a first element size in the azimuth dimension and a second elementsize in the elevation dimension. In a preferred embodiment, at least oneof the first and second element sizes is at least twice a characteristicwavelength of a waveform used to drive the array of transducer elements,where the characteristic wavelength is defined as the wavelengthcorresponding to a center frequency of the waveform.

In a particular embodiment, an ultrasound imaging transducer in a systemin accordance with the present invention exploits 1.75D elementaltechnology (Puyun Guo, Shikui Yan and Quing Zhu, “Elevation BeamformingPerformance of a 1.75D array”, IEEE 2001 Ultrasound, Ferroelectronicsand Frequency Control Conference).

An ultrasound imaging transducer in a system in accordance with thepresent invention can be manufactured in a conformal form factor, and beused as a building block (i.e., a 2D transducer array module) inultrasound blanket systems as disclosed in U.S. Pat. No. 5,666,953 andits progeny.

The present invention is also directed to a method of generating imagedata in a scanning process, using a CAC-BF (see below) technique in atleast one of an azimuth dimension and an elevation dimension, to form anultrasound image line, image plane, or image data cube. The CAC-BFmethod can be applied advantageously to any 1D or 2D ultrasonic probe orarray, and is not restricted to the preferred embodiments disclosedherein.

The present invention includes a novel beamforming method (CAC-BF) thatproduces high-resolution ultrasound images more efficiently thanconventional methods. CAC-BF divides the transducer into a number ofsmaller subapertures, each of which transmits and receives a number oflow-resolution beams that span the imaged region. High resolution isobtained at each image point by coherently combining the beamformedsignals from the subapertures, synthesising a large aperture focussed atthe point.

The present invention provides practical, clinically useful,high-resolution, 3D ultrasound, electronic, volume data acquisition.

An ultrasound imaging system in accordance with the present inventionexhibits 3D imaging with voxel resolution equal to or better than thatof state-of-the-art planar images, in both the azimuth and elevationdimensions. In one preferred embodiment, the voxel resolution is twiceas good in azimuth and/or elevation as that in state-of-the-art planarimages.

An ultrasound imaging system in accordance with the present invention isrelatively inexpensive to manufacture. The imaging system can beimplemented as an inexpensive upgrade to existing premium ultrasoundsystems, or as a stand-alone solution of comparable cost tostate-of-the-art 2D ultrasound systems.

In a particular embodiment, an ultrasound system in accordance with thepresent invention is able to electronically acquire the 3D data cube ofsize 26 mm×26 mm by 70 mm spanned by the transducer in under one second.

As with any new imaging technology, it's usefulness must be provenclinically using one or more clinical applications. The initial targetapplication of the present invention is the carotid artery, althoughthere is nothing that would restrict its use in other applications (e.g.obstetrics and gynaecology). This clinical application involvesdiagnosing plaque in the carotid artery, which can be fatal if leftuntreated. Presently, high frequency (7.5 MHz), wideband linear arrays(that use sequenced azimuth scanning) are used primarily for ultrasoundimaging of the carotid artery. State-of-the-art, 1.5D probes producerectangular images that span about 3 cm (in azimuth) by 7 cm (in depth),and whose image quality is characterized by an F number of 2 in azimuthand 8 in elevation. The ability to electronically acquire a data cube(rather than just a plane) and to improve the elevation resolution to anF number of 4 are highly desirable for this application. The presentinvention delivers such improvements affordably.

It is obvious to one skilled in the art that there is more to building a2D or 3D ultrasound imaging system as contemplated herein than simplyemploying the disclosed 2D arrays or CAC-BF method. The design andimplementation of a complete probe, beamformer or ultrasound imagingsystem assumes a large amount of hardware, software, and systemsengineering and manufacturing knowledge known to those skilled in theart. For example, the 2D probe array technology disclosed hereinrequires array manufacturing, power, switching electronics, cabling, andhousing considerations to be determined for a particular implementation.When the CAC-BF method is applied to a 1D probe array or 2D probe arrayas disclosed herein, special switching and/or cabling considerationsknown to those skilled in the art are needed. For each desired beam,contiguous sets of elements associated with the subapertures used by theCAC-BF method must be switched or electrically connected to the cablingwhich feeds the received signals to the receive electronics andbeamformers. Particular probe embodiments, all within the scope of thepresent invention, are realized by employing combinations of switchingand multiplexing electronics known to those skilled in the art, totrade-off cost, performance and complexity of the resulting probe.Switching and multiplexing electronics may be contained entirely withinthe probe housing, or distributed between the probe housing and theultrasound engine containing the receive electronics, without departingfrom the spirit and scope of the present invention. While it is apreferred embodiment of the present invention for the CAC-BF coarse andfine beamforming operations to be performed digitally in the ultrasoundengine, this functionality can also be distributed throughout the entireultrasound system, and be implemented in hardware or software in avariety of ways known to those skilled in the art, without departingfrom the scope of the present invention. In addition, post-beamformingoperations such as vector processing and imaging processing are alsoknown to those skilled in the art, and any conventional form of theseoperations could obviously be used effectively with the disclosedinventions.

Comparison of Invention with Prior Art

While U.S. Pat. No. 6,106,471 “Procedure for an Examination of Objectsby the Means of Ultrasound Waves” describes a practical solution to 3Dultrasound imaging, it is fundamentally different from the presentinvention. The present invention uses electronic scanning in both theazimuth and elevation dimensions, affording higher 3D image quality overthe mechanical, elevation scanning solution provided in U.S. Pat. No.6,106,471.

The probe or imaging transducer of the present invention is similar tothe 2D array disclosed in U.S. Pat. No. 6,238,346 in that 3D volumeacquisition is done electronically, and both attempt to reduce thenumber of transducer elements without degrading sidelobe performance.However, there are several significant differences. The 2D array in U.S.Pat. No. 6,238,346 uses square elements which are spaced sparsely in theazimuth dimension which reduces sensitivity and increases grating lobes.The present invention does not use a sparse arrangement of elements.Rather, it uses rectangular elements whose size in azimuth results in asimilar reduction in number of elements, without reducing sensitivity.The imaging transducer of the present invention has switching circuitryto form subarrays of transducer elements, each consisting of acontiguous subset of azimuth elements and a contiguous subset ofelevation elements. Adjacent subarrays in the azimuth dimensiontypically require an overlap of about 50% of the number of azimuthelements in the subarray for optimal performance. The 2D array in U.S.Pat. No. 6,238,346, on the other hand, forms subarrays by switching insubsets of contiguous elements in the azimuth dimension, but using allof the elements in the elevation dimension. Furthermore, adjacentsubarrays in azimuth are only shifted by a single element (i.e. theyrequire a much greater overlap than 50%). The imaging transducer arrayof the present invention preserves the state-of-the-art, B-mode planarimage; whereas the 2D array in U.S. Pat. No. 6,238,346 does not.

The present invention, like that described in U.S. Pat. No. 6,419,633“2D Ultrasonic Transducer Array for Two Dimensional and ThreeDimensional Imaging” is concerned with a 2D electronic transducer arraywhich can be configured to support both 2D imaging and 3D imaging.However, the present invention performs differently in three key ways asa result of its CAC-BF method and its preferred transducer design.First, it not only preserves 2D planar image quality (as compared tothat afforded by optimized 1D arrays), it provides an azimuth resolutionthat is significantly better. Second, it does not use a 2D sparse arrayfor 3D imaging; rather, it uses a full array; hence, the presentinvention does not suffer reduced sensitivity, and grating lobes areavoided by using sequential scanning in elevation when larger elements(greater than λ) are used. Finally, while the invention disclosed inU.S. Pat. No. 6,419,633 does reduce the number of signal leads (andhence channels) otherwise required, the reduction is not sufficient forthe objectives of the present invention. For example, an instantaneousaperture to achieve F/2 in azimuth and F/4 in elevation with aconventional element size λ requires a fully populated array of at least128-by-64 elements. Using the sparse 2D configuration of U.S. Pat. No.6,419,633 still requires 64×32=2,048 signal leads and channels, which isan order of magnitude larger than that needed by the present invention.As a result, the present invention is better suited to premium imagequality applications requiring larger apertures.

In U.S. Pat. No. 6,482,160 “High Resolution 3D Ultrasound Imaging SystemDeploying a Multidimensional Array of Sensors and Method forMultidimensional Beamforming Sensor Signals”, Stergiopoulos andDhanantwari describe a method for processing the signals recevied from a2D electronic, ultrasonic array. Both the assumed transducer array, andthe processing method employed are fundamentally different from thepresent invention. The assumed transmit sensor is a single, low-gaintransducer element which illuminates the entire region being imaged,causing ultrasound energy to reflect back towards a receiving array. Theregion being imaged is assumed to be in the far-field of the receivearray, and a conventional, 2D electronic scanning receive array isassumed. In the case of the present invention, the transmit aperture isa high-gain aperture (made up of several receive elements), and for thecase of the preferred CAC-BF method disclosed herein, the same apertureis used for each transmit/receive scan-line pair. The region beingimaged can be in the near-field of the receiving array of the presentinvention (which is the case for the carotid artery applicationdisclosed herein). The array element feature sizes are not conventionalfor the disclosed, preferred 2D array transducer. The beamforming methodused in U.S. Pat. No. 6,482,160 is very different that the CAC-BF methodof the present invention. First, the CAC-BF method works very well whenthe object being imaged is in the near-field of the receiving array, butthe method of U.S. Pat. No. 6,482,160 only applies to far-field imaging.It employs adaptive beamforming algorithms to increase spatialresolution over that obtainable from conventional beamformers; however,the inventors acknowledge that their beamformer's performance candegrade significantly if the assumed noise characteristics areinaccurate. The CAC-BF method of the present invention also increasesspatial resolution; however, it makes no assumptions about the noisecharacteristics and hence is more robust.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of a prior-art 1D linear arrayof ultrasonic scanning sensing elements.

FIG. 1B is a schematic perspective view of a prior-art 1D curved arrayof ultrasonic scanning sensing elements.

FIG. 2A is a schematic perspective view of the beam produced by aprior-art 1D linear array of ultrasonic scanning or sensing elements.

FIG. 2B is a schematic perspective view of the beam produced by aprior-art 1.5D linear array of ultrasonic scanning or sensing elements.

FIG. 3A is a schematic perspective view of a prior-art 1.5D linear arrayof ultrasonic scanning or sensing elements with sequential scanning inazimuth.

FIG. 3B is a schematic perspective view of a prior-art 1.5D linear arraywith phased-array scanning in azimuth.

FIG. 4 is a schematic perspective view of a 2D ultrasonic transducerarray pursuant to the present invention.

FIGS. 5A and 5B are a diagram illustrating basic CAC-BF conceptsutilized in carrying out the present invention.

FIG. 6 is a block diagram showing functional components of an ultrasoundscanning system in accordance with the present invention.

FIG. 7 is a block diagram showing elements of a fine beamformer shown inFIG. 6.

FIG. 8 is a block diagram similar to FIG. 7, showing an alternativeconfiguration of the fine beamformer of FIG. 6.

FIG. 9 is a block diagram similar to FIGS. 7 and 8, showing anotheralternative configuration of the fine beamformer of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows an ultrasound transducer probe where scanning occurs inboth the azimuth dimension (i.e., horizontally) and in the elevationdimension (vertically). The probe includes a rectangular array oftransducer elements 36 mounted to a holder or substrate member 38. Fourdifferent beams 40 a, 40 b, 40 c, 40 d are illustrated in FIG. 4,demonstrating that the probe is capable of illuminating a volume. Thetransducer elements 36 can be controlled to effect sequential scanningin azimuth and elevation as illustrated in FIG. 4; but nothing preventsone from using phased-array scanning or CAC-BF (coherent aperturecombining beamforming) scanning (see below) in azimuth and/or elevation.If phased array scanning is used in a given dimension, the acquired datausually is scan converted (i.e. transformed or mapped onto a Cartesiongrid) in order to present the image on a conventional display (i.e. amonitor using a cathode ray tube (CRT)). The net effect of any of thesescanning techniques is that a volume of ultrasound data is ultimatelyacquired electronically, which ultimately can be represented on aCartesian (x-y-z) grid.

A preferred embodiment of the ultrasound transducer device or probe ofFIG. 4 for 3D imaging of the carotid artery is characterized by thefollowing baseline parameters:

256×40 piezoelectric transducer elements 36

0.2 mm (λ)×0.8 mm (4λ) element spacing in azimuth and elevation,respectively

Scans in azimuth and elevation

Uses 128-elements for azimuth instantaneous aperture

Uses 20% elevation instantaneous aperture (i.e. 8-element subaperture)

TX focal depth is 50 mm

Imaging depth is typically 0 to 7 cm

Nominal azimuth resolution of F/2

Nominal elevation resolution (using 8 elevation elements per beam) ofF/8

High elevation resolution (using 16 elevation elements per beam) of F/4

Frequency 7.5 MHz (central wavelength 0.2 mm)

Pulse length 0.4 mm (0.27 μs) with Hann weighting, yieldingapproximately 100% bandwidth

128 digital receive channels

image volume 25.6 mm×25.6 mm×70 mm

It should be noted that any of the above parameters, including thefrequency, can be changed for other applications in order to createother preferred embodiments, and such changes would not depart from thespirit or scope of the probe in accordance with the present invention.

An ultrasound transducer device or probe, as shown in FIG. 4,characterized by these parameters produces an image data cube ofdimension 25.6 mm (elevation) by 25.6 mm (azimuth) by 70 mm (depth).(The dimension of the data cube can change without changing the basicdesign of the 2D ultrasound transducer device or carotid-scanningprobe.) The 2D ultrasound transducer device or carotid-scanning probeoperates at 7.5 MHz so the wavelength, λ, is nominally 0.2 mm. Theultrasonic carotid artery scanner therefore uses elements that arenominally spaced (and sized, neglecting kurfs) 4λ (elevation) by λ(azimuth). A key feature of the ultrasound transducer device orcarotid-scanning probe is the large elemental size in elevation.Sequential scanning is assumed to scan in the elevation dimension. The λspacing in azimuth supports both sequential scanning, as well asphased-array type scanning. Phased array scanning is usually limited toabout +/−45 deg to avoid grating lobes.

Consider the case of sequential scanning in azimuth, where 128 elementsare used at any one time so the instantaneous azimuth aperture is128×0.2 mm=25.6 mm. At 50 mm depth, this leads to an F/2 which isdesired. Since it is desired that the image extent in azimuth is also25.6 mm, 256 elements are needed in total, where 128 of them are usedfor any given vector. In elevation, the total array spans 40*0.8 mm=32.0mm. However, a sub-aperture of 8 elements (consistent withstate-of-the-art 1.5D arrays) spans 6.4 mm. Elevation vectors that arenot degraded due to asymmetries must not be closer than 3.2 mm (6.4/2)from the edge of the ultrasound transducer device or carotid-scanningprobe. Therefore the useable elevation dimension is 32.0 mm−2*3.2=25.6mm.

The azimuth dimension of stated volume can be larger than 25.6 mmbecause on transmit, one generally only uses an F/5 (i.e. about 50elements), instead of the 128 elements used on receive. One should alsonote that if phased-array scanning is used in azimuth, then only 128elements are needed in azimuth (rather than 256 as for sequentialscanning).

The aforementioned 2D transducer device of FIG. 4 has several advantagesover the conventional 2D array described earlier. The conventional 2Darray requires at least four times the number of elements to scan thesame volume (assuming the azimuth and elevation element sizes are bothλ). If a conventional probe restricts itself to the same number ofelements as the 2D transducer device disclosed herein, then it will alsoresult in lower spatial resolution and image quality because thephysical aperture will be smaller. As a result, the 2D transducer devicein accordance with the present invention is more practical, less complexand less expensive than a similarly performing conventional 2D array.

For practicality and affordability, the preferred embodiment of thepresent invention uses only say 128 receive channels, consistent withthat found in premium 2D ultrasound systems. To form beams withstate-of-the-art resolutions of F/2 in azimuth and F/8 in elevationusing the probe of FIG. 4, then 128×8=1024 elements are needed in theformation of each beam. If a higher elevation resolution is desired, sayF/4, then 16 elevation elements are needed and hence 2,048 elements toform each beam. This poses a problem, since one is restricted to only128 receive channels. Conventionally, synthetic aperture methods wouldbe used to acquire the received signals, 128 elements at a time, usingat least 8 and 16 shots, respectively, for the F/8 and F/4 beams. Thenumber of shots would be even larger if a synthetic aperture mode isalso required on transmission. The problem with using the syntheticaperture approach for the 2D probe of FIG. 4 is that the acquisition orframe time associated with acquiring the desired 3D volume increasesproportionately to the number of shots required. As it is an object ofthe present invention to also reduce the acquisition or frame time, theCAC-BF method has been developed for this purpose. This method resultsin significant reductions in acquisition time, while not degrading imagequality.

CAC-BF scanning, discussed in detail below. forms part of the presentinvention. It is used with the probe of FIG. 4, or any other 1D or 2Dprobe, when it is necessary to minimize the volumetric or imageacquisition time. The baseline CAC-BF configuration is a preferredembodiment of the CAC-BF method which uses sequential scanning inelevation, combined with CAC-BF in azimuth. For F/2 in azimuth and F/8in elevation, the CAC-BF subapertures are typically of dimension16_(az)×8_(cl) (i.e. each subaperture has 16 contiguous elements inazimuth and 8 in elevation for a total of 128), and twenty (20)subapertures are used in total to span the 128 element azimuth aperturedesired, where adjacent, azimuth subapertures have an overlap of 10azimuth elements. Typically 10 phased-array type beams are formed foreach subaperture, and the collection of resulting beams from allapertures are coherently combined to produce the final image. If higherF/4 elevation resolution is desired, then a total of forty-one (41) 8az×16_(el) subapertures are typically used, with adjacent subapertureshaving an overlap of 5 elements in azimuth. For a given application, thesize and number of the subapertures, their overlap, and the number andspacing of beams formed per subaperture are optimized to yield therequired performance.

Coherent Aperture Combining Beamforming: In General

In a preferred embodiment, the 2D ultrasound transducer device orcarotid-scanning probe of FIG. 4 uses a novel beamforming techniquereferred to as coherent aperture combining beamforming (CAC-BF) toachieve substantially reduced volume acquisition times while maintainingimaging performance.

The general concept underlying CAC-BF will now be described withreference to FIG. 5A. A volume 42 to be imaged is divided into aCartesian grid of points (voxels), nominally separated by the achievableresolution in each dimension. A number of subapertures 44 _(a), 44 _(b),. . . 44 _(n) are defined within the ultrasound transducer device orcarotid-scanning probe. A number of beams 46 _(a1), 46 _(a2), . . . 46_(ai), 46 _(b1), 46 _(b2), . . . 46 b _(j),. . . 46 _(n1), 46 _(n2), . .. 46 _(nk) from each subaperture 44 _(a), 44 _(b), . . . 44 _(n) aredirected towards different angles spanning the volume of interest 42.High-resolution voxels are formed by summing signals from a number oflow-resolution (coarse) beams originating from different subapertures 44_(a), 44 _(b), . . . 44 _(n). High resolution is achieved because thesummation results in the full aperture (i.e. the total extent of thesubapertures) being synthesised and focussed at each voxel in the image.

The differences between a method and apparatus using conventionaltechniques and a method and apparatus utilizing CAC-BF are as follows.Conventional techniques are organized into two categories: those usingsynthetic aperture beam formation, and those not. We consider the latterfirst. In a conventional ultrasound (without synthetic aperture beamformation), beams are fully formed, both on transmit and receive, with asingle shot. The number of elements in the aperture is thus limited byhow many channels are available. The transmit aperture is restricted tobe smaller than the receive subaperture to provide a reasonable depth offocus. This means that with the receive hardware currently available(i.e. we do not want to increase the number of receive channels inpremium 2D ultrasound systems), high-resolution 2D electronic scanningis not practical. The CAC-BF method differs from high-resolutionconventional beamforming in two key ways. First, conventionalbeamformers do not use subapertures 44 _(a), 44 _(b), . . . 44 _(n) inthe beamforming process. Second, conventional beamformers transmit on asmaller aperture than they receive on.

In conventional systems with ‘synthetic aperture’ beamforming, a numberof subapertures (offset in azimuth only) are focussed along a givenrange line on consecutive shots, and then the return signals are summedin order to synthesize a single beam. If a high-resolution transmit beamis also needed, then multiple subapertures are used on transmit for eachreceive subaperture; shots that have different transmit and receivesubapertures are ‘cross-terms’ in the summation. Interestingly,cross-terms actually decrease resolution, but do help to reducesidelobes. Current premium ultrasounds, when operating in syntheticaperture mode, typically use only two subapertures on receive (There isusually one direct transmit/receive and one cross-term transmit/receive.For example, transmit on a central subaperture and receive on centralsubaperture, followed by transmit on central and receive on outersubaperture.). Several key differences exist between the CAC-BF methodand synthetic aperture beamforming. First, there are no cross-terms inthe baseline CAC-BF concept as used herein, meaning that resolution isbetter for CAC-BF. Second, whereas synthetic aperture beamformers form asingle high-resolution beam for each voxel, the CAC-BF method formsseveral coarse, low-resolution beams and combines them at each voxel.Third, in a preferred embodiment of CAC-BF, each low-resolution beam isformed using transmit and receive subapertures that are the same size.In synthetic aperture beamforming, the transmit and receive subaperturesused for beam formation are different in size, the transmit beingsmaller.

In a given region, the CAC-BF concept contemplates that eachlow-resolution beam is transmitted and received from the same(sub)aperture. High-resolution ‘beams’ are not really formed; rather ahigh-resolution aperture is synthesized at each voxel. Multiplelow-resolution beams from each subaperture cover a region spanning manybeamwidths. The ultrasound imaging process utilizing CAC-BF breaks upthe existing large aperture.

A conventional ultrasound may also transmit multiple shots to get betterdepth of field (one shot for each range interval). This is because thetransmit beam must be focussed at a particular range, and only voxelsfor ranges within its depth of focus can use the beam. To cover a widerange swath, a sequence of shots are transmitted along each range line,each focussed at a different range. This forces the system to take moretime to cover the volume. The higher the resolution, the smaller is thedepth of focus, and hence the longer it takes to image a volume. Withthe disclosed CAC-BF method, the depth of focus is defined by theresolution of the coarse beams, giving it a natural advantage overstate-of-the-art beamforming methods. This is a fundamental differencebetween conventional beamforming methods and the CAC-BF method. Thepreferred embodiments of the CAC-BF described herein only require asingle shot per range line or image vector.

The ultrasound transducer device or carotid-scanning probe (see FIG. 4)is divided or partitioned into overlapping subapertures 44 _(a), 44_(b), . . . 44 _(n) (FIG. 5A). These are composed of (say) 64 to 128elements to match the available number of signal receive channels. In apreferred embodiment, the subapertures 44 _(a), 44 _(b), . . . 44 _(n)overlap by at least 50% of their width in each dimension (azimuth orelevation) where CAC-BF is applied. The percentage overlap stronglyaffects the impulse response of the resulting, high-resolution, CAC-BFimage. Grating lobes may result if enough overlap is not selected. FIG.5 shows a 1D array partitioned along the azimuth dimension intosubapertures 44 _(a), 44 _(b), . . . 44 _(n). CAC-BF is illustratedbelow for this single azimuth dimension. Extension of CAC-BF applied totwo dimensions (i.e. azimuth and elevation) is straightforward.

Coherent Aperture Combining Beamforming: Image Partition

The image space 42 is divided into overlapping beams 46 _(a1), 46 _(a2),. . . 46 _(ai), 46 _(b1), 46 _(b2, . . . 46b) _(j), . . . 46 _(n1), 46_(n2), . . . 46 _(nk) from each subaperture 44 _(a), 44 _(b), . . . 44_(n). FIG. 5A shows the beam boresights as lines originating from thesubapertures 44 _(a), 44 _(b), . . . 44 _(n), and travelling through thevolume 42. Each subaperture 44 _(a), 44 _(b), . . . 44 _(n) transmitsand receives a respective sequence of overlapping (coarse) phased-arraybeams 46 _(a1), 46 _(a2), . . . 46 _(ai), 46 _(b1), 46 _(b2), . . .46b_(j), . . . 46 _(n1), 46 _(n2), . . . 46 _(nk), each beam beingfocussed at a different angle. A pulse (shot) is transmitted and a rangeline is received for each beam 46 _(a1), 46 _(a2), . . . 46 _(ai), 46_(b1), 46 _(b2, . . . 46b) _(j), . . . 46 _(n1), 46 _(n2), . . . 46_(nk) from each subaperture 44 _(a), 44 _(b), . . . 44 _(n). The beamsfor each subaperture are normally spaced so that they cross atapproximately their −3 dB points. To avoid grating lobes, the totalangle (volume) spanned by the beams 46 _(a1), 46 _(a2), . . . 46 _(ai),46 _(b1), 46 _(b2, . . . 46b) _(j), . . . 46 _(n1), 46 _(n2), . . . 46_(nk) from each subaperture 44 _(a), 44 _(b), . . . 44 _(n) is usuallylimited by the reciprocal of the element spacing weighted by a constantthat takes into account unit conversion. This consequently limits thesize of the full aperture that can be synthesized. Only beams thatintersect the imaged volume 42 need be transmitted, thereby savingadditional acquisition time; thus subapertures at the edges of thevolume transmit fewer beams.

Coherent Aperture Combining Beamforming: Image Formation

As illustrated in FIG. 6, a two-dimensional array 48 of transducerelements mounted to a probe (not shown in FIG. 6) is accessed byswitching electronics 50. Switching electronics 50 includes a signalgenerator 52, a control unit 54 and a switching network 56. Signalgenerator 52 produces a waveform having a characteristic ultrasoundfrequency that is directed to elements of transducer array 48 byswitching network 56 in response to signals from control unit 54.Switching electronics 50 selectively energizies the elements of array 48and selectively polls those elements to effectively divide the array,along at least one of two dimensions, into subapertures 44 _(a), 44_(b), 44 _(n). As discussed above, each subaperture 44 _(a), 44 _(b), .. . 44 _(n) transmits and receives a respective plurality oflow-resolution ultrasound beams 46 _(a1), 46 _(a2), . . . 46 _(ai), 46_(b1), 46 _(b2), . . . 46 b _(j), . . . 46 _(n1), 46 _(n2), . . . 46_(nk) that span the volume 42 to be imaged. A signal processor 58 isoperatively coupled to the switching electronics 50 for coherentlycombining received beamformed signals from the subapertures 44 _(a), 44_(b), . . . 44 _(n) and synthesizing, from the coherent combination, alarge aperture focused at each point of the image volume 42.

The image formation process loops on beams 46 _(a1), 46 _(a2), . . . 46_(ai), 46 _(b1), 46 _(b2), . . . 46b_(j), . . . 46 _(n1), 46 _(n2), . .. 46 _(nk) and subapertures 44 _(a), 44 _(b), . . . 44 _(n) (2 nestedloops), collecting range lines (the sampled signal in range). From eachshot, return signals are received from the transducer elements of thetransmitting/receiving subaperture 44 _(a), 44 _(b), . . . 44 _(n).These signals are digitized by a digitizer 60 (FIG. 6) and (coarse)beamformed by a module 62, with dynamic focussing along the radial linefrom the phase center of the subaperture through the transmit focalpoint, as is usually done.

In this preferred embodiment, the next operation performed on each lineis range filtering, performed by a range filter 64. This operation islinear for the fine beamforming step to work optimally, and it retainsthe complex-valued nature of the signal; i.e. the output remains complex(I and Q). A conventional bandpass filter can be applied (matching thewaveform bandwidth), or alternatively, a matched filter can be used andapplied to the ultrasound signals; in this case, a preferred approach isto base the matched filter on the pulse replica (as the real part of thekernel) and its Hilbert transform (as the imaginary part). Matchedfiltering with the transmit pulse is logically done after coarsebeamforming, but before fine beamforming, since fine beamforming (module66) removes the range lines. Range filtering may also be omitteddepending on the waveform used.

One advantage of the way coarse beamforming and range filtering isperformed is that a 2D ultrasound engine could be used for theseoperations, making the preferred 2D probe and the CAC-BF method amenablefor upgrading existing premium ultrasound systems.

The resulting coarse beams are transferred to the fine beamformingmodule 66. Coarse beamformer module 62 and fine beamformer module 66 maybe realized by generic digital processor circuits modified by respectiveprogramming algorithms to accomplish the respective beamformingoperations.

A conventional ultrasound also loops on beams in a similar manner, butour invention uses a unique set of different beams, differing in boththe elements used and the focused directions. With conventionalultrasound, each image point is typically generated from the nearesthigh-resolution beam which is generated from one or more shots. With theCAC-BF method on the other hand, each image point is generated from anassociated set of nearby low-resolution beams, each generated from anassociated shot.

Coherent Aperture Combining Beamforming: Fine Beamforming

The image space is divided into a high-resolution grid of voxels. Thevoxels are spaced more finely than the coarse beams 46 _(a1), 46 _(a2),. . . 46 _(ai), 46 _(b1), 46 _(b2), . . . 46b_(j), . . . 46 _(n1), 46_(n2), . . . 46 _(nk), nominally at the achievable resolution incross-range from the synthesized apertures. In FIG. 5B two beamboresights (range lines) 46 _(a1), 46 _(a2) and 46 _(b1), 46 _(b2) fromeach of two subapertures 44 _(a) and 44 _(b), along with four gridpoints 72 _(a), 72 _(b), 72 _(c), 72 _(d), are shown. The intensity ateach image point (voxel) 72 _(a), 72 _(b), 72 _(c), 72 _(d) is thecoherent sum of signals received from the various nearbysubaperture-beam shots. The sum is from subapertures across the array,thereby synthesizing a larger aperture. From a given subaperture, eachvoxel's sum preferably includes the two nearest beams that straddle thegiven voxel 72 _(a), 72 _(b), 72 _(c), or 72 _(d). For example, forvoxel 72 _(d) and subaperture 44 _(a), the two beams 46 _(a1) and 46_(a2) are used. More particularly, signal sample points 73 and 73′ areused. The spatial interpolation weight for each of the two beams is suchthat the pattern of the interpolated beam reaches a maximum (peaks) atthe voxel. For image points between the beams, this not only helps thesignal-to-noise ratio, but it also reduces the sidelobe pattern of theinterpolated beam. Time-interpolation of the signal sample from eachshot is also preferrably included in the summation at each voxel, asillustrated in FIG. 5B. Various methods for spatial and temporalinterpolation known to those skilled in the art all fall within thescope of the fine beamforming method, as does using a different numberof beams or time samples for interpolation. FIGS. 7 and 8 illustratepossible modular combinations of a spatial interpolator 74, a temporalinterpolator 76, and an adder 78. The result of applying theaforementioned fine beamforming algorithm is that the signal samplesfrom a scatterer at the voxel all peak and add up in phase when summed.The range delay (which determines the phase) for a given voxel's signalsample is equal to the range delay between the subaperture phase centerand the voxel. Line 68 (FIG. 5B) represents points with approximatelythe same range delay to voxel 72 _(d) from subaperture 44 _(a) and line70 represents points with approximately the same range delay to voxel 72_(d) from subaperture 44 _(b).

Another preferred embodiment of the fine beamforming algorithm isdepicted schematically in FIG. 9. The coarse-resolution range lines fromeach subaperture 44 _(a), 44 _(b), . . . 44 _(n) are scan-converted by amodule 80 (using conventional scan conversion algorithms known to thoseskilled in the art) to the high-resolution Cartesian grid of voxels toform a low-resolution subaperture image. In this operation, the complexnature of the signal (amplitude and phase) are retained. The subapertureimages are added together by a module 82 to synthesize a largeraperture, and result in the final, high-resolution image. This additionoperation can be with unity weights, or alternatively, non-unity weightsto effect a taper. One advantage of this embodiment is thatlow-resolution images can be created quickly at a high frame rate.Higher and higher-resolution images can be obtained by using more andmore subapertures and combining their respective low-resolution images.

The high-resolution image, once formed using CAC-BF as described above,can be further processed and/or transformed using image processingmethods known to those skilled in the art. The image can be rectified(i.e. converted to an amplitude or power) or its real and/or imaginaryparts can be processed.

A key advantage of using a CAC-BF approach is that high-resolution beamsare obtained in less time for given number of (element) channels. Thisadvantage is demonstrated in the discussion that follows.

A problem with conventional ultrasound beamforming, when applied to 2Dscanning, is that it can take multiple (synthetic aperture) shots toform a high-resolution beam. This happens when the beam requires moreelements than there are channels. This can make acquisition timeunwieldy, especially for 3D ultrasound. For example, it takes at least52 seconds to image a volume of 25.6 mm by 25.6 mm by 7 cm usingλ-spaced elements in azimuth and elevation; and at least 13 seconds when4λ-elements rather than λ-sized are used in the elevation dimension,when only 128 channels are available. This assumes sequential scanningin both azimuth and elevation with a beam step of λ in azimuth and 2λ inelevation, and an F/4 elevation resolution.

Consider first the λ-spaced-element case in azimuth and elevation. Inthis case, in order to support the aforementioned undistorted volumewith F/2 in azimuth and F/8 in elevation (F/4 in elevation results in anundistorted elevation volume dimension that is less than 25.6 mm), thearray is 256λ by 160λ with 256 elements in azimuth and 160 in elevation,for a total of 40,960 elements. For F/4 and F/8 in elevation, thereceive subaperture has 128×64=8,192 elements and 128×32=4,096 elements,respectively. The number of beams required to sample the volume is also128×64=8,192 assuming a beam step of λ in azimuth and 2λ in elevation.Using a 10 kHz firing rate (suitable for 7 cm depth), the minimumacquisition time (i.e. with 1 shot per beam) is 0.8192 s, assuming that8,192 receive channels and 4,096 receive channels are available,respectively, for F/4 and F/8 elevation resolution. Of course, buildinga system with these many channels is extremely expensive and impracticaltoday. If only 128 channels are available (i.e. as found in modernpremium systems), then at least 8,192 elements/128 channels=64shots/beam and 4,096/128=32 shots/beam are needed, causing theacquisition times to increase to 52.4 s and 26.2 s, for F/4 and F/8respectively. These numbers assume a single transmit focus. If multipletransmit focii are used (up to 4 are used in practice), then theacquisition times increase proportionately.

Consider next the case where 4λ-elements rather than λ-sized are used inthe elevation dimension. Then the number of elevation elements per beamreduces to 16 and 8, for F/4 and F/8, respectively. As a result, thenumber of elements in each receive subaperture reduces to 2,048 and1,024 respectively. If 2,048 and 1,024 receive channels are availablefor the respective F/4 and F/8 elevation resolutions, then theacquisition time is again 0.8192 s. With only 128 available channels,however, this acquisition time increases to 13.1 s and 6.55 s,respectively. Once again, these times increase proportionately with thenumber of transmit focii used.

With the present CAC-BF method, low-resolution beams 46 _(a1), 46 _(a2),. . . 46 _(ai), 46 _(b1), 46 _(b2, . . . 46b) _(j), . . . 46 _(n1), 46_(n2), . . . 46 _(nk) are used, needing fewer elements per beam, so thatpotentially only one shot is needed per beam. The low-resolution beams46 _(a1), 46 _(a2), . . . 46 _(ai), 46 _(b1), 46 _(b2, . . . 46b) _(j),. . . 46 _(n1), 46 _(n2), . . . 46 _(nk) cover a greater volume, butbeams are needed from more (smaller) subapertures 44 _(a), 44 _(b), . .. 44 _(n) in order to get resolution. The volume is covered by the‘product’ of subapertures 44 _(a), 44 _(b), . . . 44 _(n) and beams 46_(a1), 46 _(a2), . . . 46 _(ai), 46 _(b1), 46 _(b2), . . . 46b_(j), . .. 46 _(n1), 46 _(n2), . . . 46 _(nk). This can be accomplished with manysubapertures, and few beams per subaperture (coarse beams, few elementsper beam), or with just a few subapertures, with many beams per (finerbeams, many elements per beam) subaperture. The product of the two (thetotal number of beams) is, to first order, independent of subaperturesize. Thus with the present method, a certain number of beams 46 _(a1),46 _(a2), . . . 46 _(ai), 46 _(b1), 46 _(b2), . . . 46b_(j),. . . 46_(n1), 46 _(n2), . . . 46 _(nk) are required to cover a given volume ata given resolution, and coverage time is proportional tovolume/resolution. Once the beams 46 _(a1), 46 _(a2), . . . 46 _(ai), 46_(b1), 46 _(b2, . . . 46b) _(j),. . . 46 _(n1), 46 _(n2), . . . 46 _(nk)are low enough resolution (i.e. they have a small enough number ofelements), they only need one shot. Thus one of the key advantages ofour method: when down to 1 shot for each beam, we have minimizedcoverage time. The aforementioned volume can now be imaged in under 1.6seconds for F/4 in elevation, and in under 0.7 seconds for F/8 inelevation. The calculations are illustrated below.

Consider first the F/4 elevation case with 4λ-elements. With 16 elementsneeded in elevation and 128 channels available, one preferred CAC-BFsolution is to use a receive subaperture containing 8 elements inazimuth and 16 elements in elevation. Using 41 subapertures 44 _(a), 44_(b), . . . 44 _(n) to span the 128 azimuth elements with a preferredoverlap of 5 elements between adjacent subapertures (in azimuth), apreferred 6 beams can be used to cover the 25 mm in azimuth. As aresult, the number of beams (shots) needed to acquire the whole volumeis 6 (per subaperture)×41 (subapertures)×64 (elevation beams)=15,744,which translates to an acquisition time of 1.57 s. Again, we haveassumed a beam step of 2λ in elevation. Furthermore, because of thelarge depth of focus associated with the CAC-BF technique, a singletransmit focus should suffice for virtually all imaging applications.This results in eight times the frame rate over the conventionalbeamforming technique, even when only a single transmit focus isemployed with conventional imaging.

Finally, consider the F/8 elevation case with 4λ-elements and 128available channels. With only 8 elements needed in elevation for eachreceive subaperture, one preferred CAC-BF solution is to use a receivesubaperture containing 16 elements in azimuth and 8 elements inelevation. With 20 subapertures overlapped by a preferred 10 elements inazimuth to span the entire 128, 10 beams are preferably needed by eachsubaperture to cover the 25 mm in azimuth. As a result, 10×20×64=12,800shots are needed which translates to 1.28 s for acquisition. The CAC-BFtechnique results in five times the frame rate over the conventionalbeamforming technique, in this case, assuming the conventionalbeamformer uses a single transmit focus. If the elevation beam step wasincreased to 4λ, then the frame time would reduce to 0.64 s usingCAC-BF.

One advantage of the present methodology is that the aperture size canbe tailored to every grid point 72 _(a), 72 _(b), 72 _(c), 72 _(d) (FIG.5B). The preferred algorithm (using all available intersecting beams)naturally uses more apertures (on both transmit and receive) for gridpoints at greater ranges. With this embodiment, azimuth resolution (inmm) is constant with range. In a conventional ultrasound, the number ofelements making up the aperture is not allowed to grow beyond the numberof available channels. In the case of the present invention, theeffective aperture can grow to the full size of the physical aperture,exceeding the number of available channels, to the extent limited by theelement directivity and desired grating lobe performance.

Another advantage of the CAC beamforming algorithm is that it can becombined with other conventional scanning approaches so that certainparts of a volume to be imaged use CAC beamforming while other parts useconventional beamforming. For example, CAC beamforming could be appliedonly at further ranges where resolution degrades and conventionalbeamforming used elsewhere.

Another advantage of the methodology described herein is that betterdepth of field is obtained with lower resolution beams. This means thatan equivalent 3D volume can be covered with fewer shots, or a greatervolume can be covered with the same number of shots.

Yet another advantage of the methodology described herein is that betterresolution is achieved with the same size aperture (because of the lackof cross-terms). Resolution is trio times better than that of areceive-only aperture (i.e. a system that uses a significantly lowerresolution transmit aperture), and 1.4 times better than that of aconventional beam using full apertures on transmit and receive. It is tobe noted that the full aperture is not normally used on transmit becauseof the limited depth of field, thus CAC-BF gets almost twice theresolution of conventional systems using the same sized physicalaperture. Analyses and experimental measurements show that CAC-BF with50% overlap performs equivalently to a conventional synthetic apertureof twice its size.

Other variations to the CAC-BF method are described to illustrate thescope of the CAC-BF method in accordance with the present invention.

CAC-BF can be performed with an arbitrary amount of subaperture overlap,recognizing that the resulting image (beam) response characteristics(e.g. the sidelobe behavior including the presence of grating lobes) atan image point will be affected accordingly.

It is to be noted that CAC-BF can be done in two dimensions, or,alternatively, CAC-BF can be performed in one dimension and conventionalscanning in the other. CAC-BF can also be used with one-dimensional(1.5D, 1.75D etc.) probes to increase the frame rate for a given numberof channels. The frame rate is further improved by the fact that thedepth of focus is greater, reducing the number of transmit focii needed.Alternatively, larger effective apertures (better resolution) can berealized without reducing the frame rate. CAC-BF can also be used andtailored to work with systems having virtually any number of receivesignal channels.

It is to be noted also that the fine grid within the image space neednot be Cartesian and that the coarse beams need not be spaced equally inangle. For example, the beams could be spaced equally in sine-space, orspaced equally in the Cartesian grid. When CAC-BF is applied in bothdimensions, the coarse beams could be placed on a grid that is not theproduct of an azimuth and an elevation grid (e.g. hexagonal orcylindrical scanning).

It is possible to include subaperture cross-terms, in order to improvesidelobes. Moreover, it is possible to transmit from one subapertureonly, and receive from the rest of the subapertures.

This has the disadvantage of only getting half the resolution per lengthof aperture, but gets equivalent resolution to CAC-BF per pulse, becauseaperture overlap is not needed. A potential advantage is a reduction inhardware complexity (may not need transmit multiplexer, or it will besimpler). The reciprocal arrangement (transmit from all subapertures,only receive from middle one) may also be attractive.

Interpolation between beams helps reduce the sidelobes at grid pointswhere the beams from different subapertures don't line up, effectivelysmoothing the addition of the subapertures at these points. Theinterpolation can be of any desired amount, using any algorithm known tothose skilled in the art. It is possible not to use any interpolationbut performance will be affected accordingly.

Shading (windowing) may be done in the summation across the aperture (toreduce sidelobes with narrow-band systems). For similar reasons, oralternatively, the subapertures themselves could be shaded or windowed.

Higher resolution coarse beams (requiring multiple shots) could beutilized, trading off coverage time versus sidelobes. Dynamic focussingmay be unnecessary if beams are of low enough resolution (i.e. verysmall subapetures), and this may reduce complexity.

The ‘product’ of beams and subapertures need not have each of thenominally 50% overlapping subapertures transmitting all of the coarsebeam angles. The product could be formed with a greater number ofhighly-overlapped subapertures, each transmitting a smaller number (e.g.one) of the beam angles. This has the advantage of having smaller blindzones at the close ranges between the subaperture centres, where nobeams are transmitted.

Non-linear or adaptive, high-resolution beamforming techniques arecomputationally expensive, but may be worthwhile in some applications tocombine the multiple subapertures used in the fine beamformingalgorithm. The structure of the CAC-BF method is appropriate as there istypically a small number of subapertures. This is not burdensome if ittakes a few seconds or minutes to compute; a physician could look at alinearly beamformed image, and suggest an area he would like to seebetter resolved; then the high-resolution algorithm could be applied.

Coherent Aperture Combining Beamforming: 2D Scanning

With 2D electronic scanning, the designer has a number of choices forwhich methods to use in each dimension. The choices include element size(n*lambda, where n can be between 0.5 and 4), what type of scanning(phased, sequenced, CAC-BF), the number of elements in the aperture, andwith CAC-BF, the subaperture sizes and overlap. As element size goes up,the cost to achieve a certain level of resolution goes down, butsidelobe performance degrades. Each element size has a maximumachievable resolution, and larger elements have poorer performance. Forexample, 4-lambda elements cannot do better than about 0.35 mm azimuthresolution, whereas lambda elements can achieve about 0.2 mm azimuthresolution at 7.5 MHz. Systems using CAC-BF in 2D with lambda or2-lambda elements in both dimensions are viable compromises. WithCAC-BF, the larger the subapertures, the costlier (i.e. more channelsare needed), but sidelobe performance is better. Once the 1D performanceof each alternative is established by testing, then 2Dcost/performance/acquisition-time trade-offs can begin. A key feature ofthe CAC-BF algorithm is that it naturally provides the designer withthis cost/performance/acquisition time trade-off.

With 2D CAC-BF, in order to deal with motion within the imaged volume,one can transmit the beams ordered within the volume, i.e. all the beamsin top left corner first, then each row left to right, rows ordered topto bottom. In this way, each grid point is illuminated over a shortperiod of time. This is exactly true only in focal plane, grid points atlonger or shorter ranges taking somewhat longer to illuminate. Thealternative of ordering by subaperture means that every grid pointrequires the whole sequence of pulses to be imaged.

The CAC-BF method has been simulated extensively and its performance asdescribed herein validated by these simulations. In addition,experimental results have been obtained by applying CAC-BF in theazimuth dimension as described herein using a 64-channel ultrasoundsystem and a 192-element off-the-shelf probe, suitably programmed toimplement the CAC-BF method. The improved resolution, the high-qualityimagery, and the reduction in acquisition time compared to conventionalbeamforming have all been confirmed. Images (2D or 3D) may be generatedon a video monitor 84 (FIG. 6) by an image generator 86 in response toimage data stored in a memory 88 connected to output of processor 58,more particularly to an output of a CAC component 90, and even moreparticularly to an output of fine beamformer 66. CAC component 90includes digitizer 60, coarse beamformer 62, range filter 64 and finebeamformer 66.

3D Ultrasound Imaging Systems

An ultrasound imaging system in accordance with the present inventionprovides a novel solution for 3D ultrasound imaging that is affordable,and yet high-performing. Unlike other designs which degradestate-of-the-art imaging performance in order to reduce cost, thepresent solution maintains or exceeds state-of-the-art imagingperformance, while keeping the 3D ultrasound system cost comparable tothat of 2D ultrasound systems.

State-of-the-art azimuth imaging performance requires an F number of 2;i.e. an F/2. The F number is the ratio of the focal range divided by theimaging aperture dimension. For example, an F/2 at 5 cm depth requiresan instantaneous receive aperture of size 2.5 cm. The present probe isdesigned to provide an F/2, and an enhanced resolution of F/1 (i.e.twice as good as state-of-the-art).

State-of-the-art-elevation imaging performance requires an F/8. Thepresent probe provides a standard elevation resolution of about F/8 andis capable of providing an enhanced elevation resolution of F/4 (twiceas good as state-of-the-art) or better.

For state-of-the-art resolutions in both azimuth and elevation, the 2Dprobe of the present invention (for 3D imaging) is able to acquire a 25mm×25 mm×70 mm volume electronically in under 1 second; the same volumeis acquired with enhanced resolutions (azimuth and elevation) in under 2seconds. And these acquisition times are achievable when just 128receive channels are available, keeping the number of channels (andhence cost) comparable to state-of-the-art ultrasound imaging systems(for 2D imaging).

This win-win (performance-cost) 3D imaging technology is made possiblefrom the use of the beamforming techniques of the present invention thatreduce acquisition time (i.e. the number of shots needed) when thenumber of received channels available is less than the number ofelements in the imaging aperture. The beamforming techniques arereferred to as coherent aperture combining beamforming (CAC-BF) asdiscussed earlier.

In order to reduce the element count and simplify the transducer design,the 2D probe pursuant to the present invention is able to exploit 1.75Delemental technology, with λ spacing in azimuth, and 4λ spacing inelevation in one preferred embodiment. As a result, its baselinetransducer requires only 10k elements, 256(λ)×40(4λ)=10,240, in order toproduce an undistorted volume extending 25.6 mm in azimuth and 25.6 mmin elevation. The beamforming is completely electronic (i.e. nomechanical lenses are used in elevation). Individual receive beams usejust 128×8=1,024 elements for standard, state-of-the-art imagingresolutions, and 128×16=2,048 elements for enhanced resolution imaging.

The 2D probe can be used with conventional beamforming algorithms, aswell as with CAC-BF, making it very versatile, saving on the number ofprobes otherwise required by an ultrasound system. For example, it canoperate as a conventional 1D array, employing conventional scanningtechniques such as sequential or phased-array scanning in azimuth (orelevation) only. If 2D scanning is desired, then conventional electronicscanning can be performed in both azimuth and elevation. For example, ifsequential beamforming is used in both azimuth and elevation, then forstandard imaging resolutions (F/2 in azimuth and F/8 in elevation) 128beams are needed in azimuth and 32 are needed in elevation to fill thevolume. For the case of 70 cm depth, an acquisition time of 6.5 secondsresults with two transmit focal ranges (128×32 vectors×8 shots/vector×2focii/10 kHz). For enhanced imaging resolutions, 64 elevation beams areneeded along with 16 shots/beam (2048 elements/128 channels) resultingin an acquisition time of 26.2 seconds.

With CAC-BF, the beamforming can be configured in a number of ways (asdescribed earlier) to reduce the volume acquisition times required ifonly conventional beamforming algorithms were used. For standard imagingresolutions, consider the case where 20 overlapped subapertures, eachconsisting of 16 (in azimuth) by 8 (in elevation) elements are used with10 shots (beams) per subaperture. CAC-BF is performed in azimuth whilesequential beamforming is performed in elevation. The acquisition timefor this configuration has already been shown to be just 0.64 seconds(10 shots/subap×20 subaps×32 elev_beams/10 kHz), a substantial reductionover the 6.5 seconds required using only sequential beamforming. Forenhanced imaging resolutions, recall the case where 41 overlappedsubapertures, each consisting of 8 (in azumith) by 16 (in elevation)elements are used with 6 shots (beams) per subaperture. In this case,the volume acquisition has been shown to take only 1.57 seconds (6shots/subap×41 subaps×64 elev_beams/10 kHz), as compared to the 26.2seconds needed by the sequential beamformer.

Although the invention has been described in terms of particularembodiments and applications, one of ordinary skill in the art, in lightof this teaching, can generate additional embodiments and modificationswithout departing from the spirit of or exceeding the scope of theclaimed invention. While the preferred embodiment described hereinrepresents the form of the invention currently being developed for itscarotid artery application, the scope of this invention goes far beyondthe form of this preferred embodiment. For example, the CAC-BF solutioncan be used in the elevation dimension, instead of azimuth, or it couldbe used in both dimensions, and still be within the scope of theinvention. Alternatively, the number and size of elements in eachdimension of the transducer could be different, and still fall withinthe scope of the invention. In the limiting case, the 2D transducerarray could collapse to be a 1D array (i.e. designed to scan in only onedimension), and if CAC-BF is used in that single dimension, thisconfiguration is still within the scope of the invention. Accordingly,it is to be understood that the drawings and descriptions herein areproffered by way of example to facilitate comprehension of the inventionand should not be construed to limit the scope thereof.

1. A method for generating image data in an ultrasound scanning process, comprising selectively energizing transducer elements in an array of transducer elements and selectively polling said transducer elements to effectively divide said array into a plurality of subapertures transmitting and receiving a plurality of low-resolution ultrasound beams that span a volume to be imaged; and coherently combining beamformed signals from said subapertures to synthesize an aperture larger than any one of said subapertures and focused at each point of said volume.
 2. The method defined in claim 1 wherein receiving a plurality of low-resolution ultrasound beams includes receiving, from said transducer elements, return signals encoding reflected ultrasound waveforms; collecting said return signals to form range lines; digitizing said return signals; and coarse beamforming the digitized signals.
 3. The method defined in claim 2 the coherent combining includes subjecting the coarse beamformed digitized signals to a fine beamforming.
 4. The method defined in claim 3 wherein at least one of said subapertures receives a plurality of said low-resolution ultrasound beams, the fine beamforming including spatially interpolating adjacent beams of said one of said subapertures.
 5. The method defined in claim 3 wherein the fine beamforming includes temporally interpolating between samples of said coarse beamformed digitized signals.
 6. The method defined in claim 3 wherein the fine beamforming includes determining range delays for signal samples.
 7. The method defined in claim 3 wherein the fine beamforming includes scan-converting the range lines to a high-resolution grid of voxels to form a low-resolution subaperture image for each subaperture, and coherently combining the low-resolution subaperture images to synthesize a larger aperture and a final high-resolution image.
 8. The method defined in claim 3 wherein the coherent combining further includes range filtering the coarse beamformed digitized signals prior to the subjecting of the coarse beamformed digitized signals to the fine beamforming.
 9. The method defined in claim 1 wherein at least one of said subapertures receives a plurality of said low-resolution ultrasound beams, the coherent combining including spatially interpolating adjacent beams of said one of said subapertures.
 10. The method defined in claim 1 wherein the coherent combining includes temporally interpolating said beamformed signals.
 11. The method defined in claim 1 wherein the coherent combining includes determining range delays for said beamformed signals.
 12. The method defined in claim 1 wherein the selectively energizing and the selective polling of said transducer elements includes energizing and polling said transducer elements so that each subaperture transmits and receives a respective sequence of overlapping phased-array beams each focused at a different angle.
 13. The method defined in claim 12 wherein said overlapping phased-array beams are spaced so as to cross approximately at respective −3 dB points.
 14. The method defined in claim 12 wherein said transducer elements have an inter-element spacing, the overlapping beams of any given one of said subapertures subtending a total angle of less than a weighted mathematical reciprocal of said inter-element spacing.
 15. The method defined in claim 1 wherein each of said subapertures overlaps an adjacent one of said subapertures, the overlap including at least 50% of the transducer elements included in said adjacent one of said subapertures.
 16. The method defined in claim 1 wherein each of said subapertures overlaps an adjacent one of said subapertures, the overlap including approximately 50% of the transducer elements included in said adjacent one of said subapertures.
 17. The method defined in claim 1 wherein said array is a two-dimensional array in an image space having a first dimension and a second dimension, said subapertures extending along at least one of said first dimension and said second dimension.
 18. The method defined in claim 1 wherein the selective energizing of said transducer elements includes energizing transducer elements of only one of said subapertures to generate at least one outgoing waveform.
 19. The method defined in claim 1 wherein the selective energizing of said transducer elements includes energizing transducer elements of each of said subapertures so that each of said subapertures generates at least one outgoing waveform.
 20. The method defined in claim 1 wherein the selective energizing of said transducer elements and the selective polling of said transducer elements are such that each of said subapertures transmits and receives a plurality of low-resolution ultrasound beams that span the volume to be imaged.
 21. The method defined in claim 1 wherein the selective energizing of said transducer elements and the selective polling of said transducer elements are such that each of said subapertures transmits an outgoing waveform while fewer than all of said subapertures receive reflected waveforms.
 22. The method defined in claim 1 wherein the selective energizing of said transducer elements and the selective polling of said transducer elements are such that fewer than all of aid subapertures transmit an outgoing waveform while all of said subapertures receive reflected waveforms. 23-43. (canceled) 